This is the second invasive Fallopia taxon recorded in Southern Chile. Fallopia sachalinensis (common name: Giant knotweed), an herbaceous perennial native to East Asia, and has been introduced in several countries (Europe, North America, and Canada) as a garden ornamental plant (Bailey & Stace 1992, Pysek & Prach 1993, Marigo & Pautou 1998). The species has many of the characteristics of an "ideal invader" (Marigo & Pautou 1998). Fallopia sachalinensis invades (sensu Richardson et al. 2000) riparian and human-made habitats and often spreads into semi-natural vegetation. It prefers low altitude alluvial plains with a constant flow of water and warm temperatures during the vegetation period of physiological activity (Marigo & Pautou 1998). The establishment of F. sachalinensis is accelerated by human activities. Rhizomes fragments being spread by ploughing and soil transport from one site to another that can serve as a starting point for new populations. Recently, Saldaña et al. (2009) reported Fallopia japonica (Houtt.) Ronse Decr. invades natural environments, but to our knowledge, F. sachalinensis has not been reported as invasive plant in Chile. F. sachalinensis is used as ornamental plant in the area recorded (Schilling 1965), and was introduced as fodder plant (Izquierdo 1912). The specimens were collected from a small population of approximately 6 m2 growing at the border of a small water channel on both side of the road without other species around it. The population had individuals from 5 cm to 2.5 m high. Some individuals possessed vegetative reproduction. The entire population showed no evidence of foliar and floral herbivory.

Fallopia sachalinensis has many characteristics that confer the ability to invade the habitats mentioned above; clonal reproductive spread (i.e. vegetative multiplication of rhizomes) associated with an extraordinary high rate of proliferation of rhizomes, abundant leaf cover (Fig. 1a), favourable leaf orientation to capture high light intensity, mechanisms for adaptation to adverse conditions and the use of competitive strategies to monopolize resources (Marigo & Pautou 1998). Additionally, the hybrid of F. sachalinensis with F. japonica, F. x bohemica has been documented to be twice as an invasive as the parents species, having a more varied genome (Mandak et al. 2004). Among the impact caused by F. sachalinensis are the restriction of access to stream and river banks, the damage to flood fence structures, the outcompeting and exclusion of native vegetation, and the decrease of potential vegetation development (Marigo & Pautou 1998).

The pathway for the introduction (i.e. ornamental) and the date of introduction of F. sachalinensis in this area probably was the same of F. japonica, which was recently recorded in the same area (Saldaña et al. 2009), because the distance between both records is only 100 km. Therefore, the chances for hybridization by cross-pollination between them are high, because seeds of F. japonica may be dispersed beyond 16 m (Tiébré et al. 2007); and in the case of plants growing next to riverbanks, there is a good chance of hybrid seeds

being dispersed downstream for much greater distances. In order to inhibit further invasions and the hybridization between both species, it is necessary to determine the area invaded by F. japonica and F. sachalinensis, and evaluate the chances and costs of eradication and control. Probably, these species are still on its primary spreading phase, because no other populations were located around the same area. Long-term establishment of a new widespread pest can be avoided using an appropriated eradication program. On the contrary, the high invasion ability of Fallopia taxa will undoubtedly result in occupation of new localities in the near future.